Wave transmission lines and networks – Coupling networks – Wave filters including long line elements
Reexamination Certificate
2001-10-26
2004-08-17
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Wave filters including long line elements
C333S0990MP, C505S210000
Reexamination Certificate
active
06778042
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2000-330615, filed Oct. 30, 2000; No. 2000-333069, filed Oct. 31, 2000; No. 2000-333070, filed Oct. 31, 2000; No. 2000-333071, filed Oct. 31, 2000; and No. 2001-095966, filed Mar. 29, 2001, the entire contents of all of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a high-frequency device, and more particularly to a microwave filter and a high-frequency device related to the microwave filter.
2. Description of the Related Art
A communication apparatus for communicating information by wireless or by wire is composed of various devices, including amplifiers, mixers, and filters. That is, it includes many devices making use of resonance characteristics. For instance, a filter is composed of a plurality of resonating elements arranged side by side and has the function of allowing only a specific frequency band to pass through. Such a filter is required to have a low insertion loss and permit only the desired band to pass through. To meet these requirements, resonating elements with high unloaded Q values are needed.
One method of realizing a resonating element with a high unloaded Q value is to use a superconductor as a conductor constituting a resonating element and further use a material whose dielectric loss factor is very small, such as Al
2
O
3
, MgO, or LaAlO
3
, as a substrate. In this case, however, the unloaded Q value is 10,000 or more and the resonance characteristic is very sharp. As a result, the desired characteristic cannot be obtained unless the resonance characteristic is adjusted with high accuracy in the design stage.
To overcome such a problem, a resonator and a filter which have the function of adjusting the resonance frequency have been proposed. Methods of tuning the frequency of a resonator or a filter include a method of providing a dielectric whose permittivity depends on the applied electric field in the vicinity of a resonating element and thereby applying a voltage to the dielectric and a method of providing a magnetic material whose permeability varies with the applied magnetic field in the vicinity of a resonating element and applying a magnetic field to the magnetic material.
For example, what has been described in reference
1
(“Electrically tunable coplanar transmission line resonators using YBa
2
Cu
3
O
7-x
/SrTiO
3
bilayers” by A. T. Findikoglu et al., Appl. Phys. Lett., Vol. 66, p. 3674, 1995) is a method of forming a coplanar resonator composed of an oxide superconductor film on an LaAlO
3
substrate whose surface is covered with a dielectric SrTiO
3
film whose permittivity depends on the applied electric field and applying a voltage between the central transmission line and the ground on both sides and thereby tuning the resonance frequency f. In this case, the tuning width &Dgr;f/f is 4%. Since a dielectric whose permittivity depends on the field strength, such as SrTiO
3
, has a high dielectric loss factor (tan &dgr;), the unloaded Q value decreases to about 200. This causes the following problem: the advantage that use of a very low loss superconductor increases the unloaded Q value disappears.
Similarly, in reference
2
(“Tunable and adaptive bandpass filter using a nonlinear dielectric thin film of SiTiO
3
” by A. T. Findkoglu et al., Appl. Phys. Lett., Vol. 68, p. 1651, 1996), a tunable band-pass filter composed of a plurality of coplanar resonators capable of performing the aforementioned frequency tuning has been described. In this case, since the unloaded Q value of each resonator constituting the filter is small as described above, the rising and falling of the frequency passband called the skirt characteristics are gentle, impairing the frequency selectivity. There is another problem: when the frequency passband is changed by the application of a voltage, the insertion loss, skirt characteristics, and ripples in the frequency passband vary.
Furthermore, Jpn. Pat. Appln. KOKAI Publication No. 9-307307 or Jpn. Pat. Appln. KOKAI Publication No. 10-51204 has disclosed a filter where a dielectric whose permittivity depends on a voltage is provided on a filter element and a pair of voltage applying electrodes is provided near the dielectric. In this case, it is possible to change the permittivity locally or distribute the permittivity according to the arrangement of electrodes or the applied voltage. This alleviates the above problem to some degree, that is, the problem of changes in the insertion loss, skirt characteristics, and ripples incidental to the tuning of the passing frequency band of the band-pass filter.
This method, however, requires not only a dielectric whose permittivity varies with the applied voltage but also voltage applying electrodes, leading to an additional loss caused by the electrodes. As a result, the unloaded Q value of a single resonator is as small as several hundred or less, which makes it impossible to obtain a filter with a sharp skirt characteristic.
Furthermore, when the tuning of the frequency is done by applying a voltage to the electrode pair and changing the permittivity of the dielectric uniformly, the loss due to the dielectric is great and in addition varies with the applied voltage. Consequently, the Q value of the resonating element constituting the filter varies as a result of tuning, which causes a problem: the insertion loss of the filter and the characteristics in the passband deviate from the desired characteristics. Moreover, this method permits the permittivity and dielectric loss factor to follow a spatial distribution and therefore cannot cause them to vary uniformly all over the surface.
Another method has been described in, for example, reference
3
(“Tunable Superconducting Resonators Using Ferrite Substrates” by D. E. Oates and G. F. Diome, IEEE MTT-S digest, p. 303, 1997). In this method, a plate of magnetic material Y
3
Fe
5
O
12
(YIG) whose permeability varies with the applied magnetic field is provided on a microstrip-structure resonator formed on a substrate. A direct-current magnetic field is externally applied to the plate, thereby tuning the resonance frequency. Although the tuning width &Dgr;f/f is 3%, almost the same as that in the aforementioned dielectric control method, the unloaded Q value has been improved and is about ten times as large as that of a dielectric-control-type resonator. However, when a plurality of resonators with such a tuning function are arranged side by side, thereby forming a band-pass filter capable of tuning the passing frequency band, the electromagnetic coupling between the resonating elements and between the resonating elements and the input and output lines varies because the passing frequency band varies according to the application of the magnetic field. This variation causes a problem: the insertion loss, skirt characteristics, and ripple characteristics of the filter deviate from the original design. Moreover, when the passing frequency band is 5 GHz or less, the insertion loss becomes greater because of the magnetic loss.
Still another method has been disclosed in Jpn. Pat. Appln. KOKAI Publication No. 5-199024. In this method, a superconductive resonator is such that a vertically movable conductor rod, dielectric strip, or magnetic material rod is provided on a resonator with a single resonating conductor and the resonance frequency can be adjusted by controlling the position of the rod. However, to apply the method to a filter where a plurality of resonating elements are arranged side by side, it is necessary to move the conductor rod or the like on each resonating element over the same distance with high accuracy. There is another problem: changing the frequency leads to changes in the characteristics within the band, such as ripples or bandwidth.
In the description of reference
4
(“On the Development of Superconducting Microstrip Filters for Mobile Communications Applications” by Jia-Sheng Hong et
Aiga Fumihiko
Fuke Hiroyuki
Katoh Riichi
Kayano Hiroyuki
Terashima Yoshiaki
Glenn Kimberly
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Pascal Robert
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